High transmittance glass sheet and method of manufacturing the same

ABSTRACT

A high transmittance glass sheet is provided that is formed of a soda-lime-silica glass composition containing, expressed in wt. %, less than 0.020% of total iron oxide in terms of Fe 2 O 3  and 0.006 to 2.0% of zinc oxide. The glass sheet allows the formation of nickel sulfide particles to be suppressed by the addition of a zinc compound to a glass raw material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a high transmittance glass sheetof soda-lime-silica glass manufactured mainly by a float process. Morespecifically, this invention relates to a high transmittance glass sheetthat allows the formation of nickel sulfide (NiS) in a process ofmelting a glass raw material to be suppressed effectively.

[0003] 2. Related Background Art

[0004] In methods of manufacturing a soda-lime-silica glass sheet suchas a float process and a roll out process, the following problem mayarise. That is, in a process of melting raw materials in a furnace at ahigh temperature near 1,500° C., metal particles of stainless steel orthe like containing nickel (Ni) may be mixed into the raw materials. Themetal particles may react with sulfur (S) in salt cake (Na₂SO₄) includedin the raw materials. As a result of this reaction, nickel sulfide (NiS)is formed as minute foreign matter in glass products. NiS particles arepresent at a minimal rate of about one per a little over 10 tons ofglass products, and are of an extremely minute spherical form having adiameter as small as about 0.3 mm or less. Therefore, it is difficult todetect NiS particles on production lines.

[0005] Some soda-lime-silica glass sheets are tempered to be used forbuildings, vehicles, cover glass plates for solar cell panels, solarwater heaters or the like. In a tempering process, a glass sheet isheated to a temperature near the softening point (about 600° C.) of theglass sheet. Then, the glass sheet is quenched so that compressivestress layers are generated in surfaces of the glass sheet.

[0006] When NiS is contained in a tempered glass sheet, NiS is presentin an α phase that is stable at about 350° C. or higher, and undergoesphase transition with the lapse of time to a β phase that is more stableat room temperature. This phase transition causes NiS particles toexpand in volume. As a result of this, micro cracks may appear in thevicinity of the NiS particles. Inside the tempered glass sheet, atensile stress layer exists, having a thickness of about two-thirds thatof the glass sheet. When cracks appear in the tensile stress layer, thecracks run rapidly to cause spontaneous fracture of the tempered glasssheet.

[0007] To prevent such spontaneous fracture of a tempered glass sheet,so-called soaking has been known. In this method, tempered glass sheetsare heated to 300° C. or lower in a furnace (soaking furnace). Then, thetempered glass sheets are maintained in the furnace for a predeterminedtime, so that NiS undergoes phase transition from an α phase to a βphase. This forces the tempered glass into breakage. In this manner,defective glass products containing NiS are eliminated.

[0008] However, operations such as the soaking in which heat treatmentis mainly performed cost considerable energy and time, thereby causingan increase in manufacturing cost. This also is a serious hindrance toshortening of delivery times and an improvement in productivity.Further, defective products are eliminated in the soaking, therebycausing a decrease in product yield.

[0009] JP 9(1997)-169537 A discloses a method of manufacturing asoda-lime-silica glass in which 0.01 to 0.15 wt. % of a zinc compoundsuch as zinc nitrate and zinc sulfate is added to raw materials, therebyallowing the formation of NiS to be suppressed.

[0010] Meanwhile, there has been a growing demand that a hightransmittance glass sheet, more specifically, a glass sheet having alight color and a high transmittance be used for an interior glass, ashowcase, a display case, a high transmittance non-colored window glass,a high transmittance non-colored mirror, a glass substrate for a solarcell panel, a cover glass plate for a solar cell panel, a solar waterheater, a material for a high solar-heat transmittance window glass, anda flat display substrate glass such as a front panel or the like.However, no high transmittance glass sheet has been known so far that issuitable for industrial mass production.

SUMMARY OF THE INVENTION

[0011] A high transmittance glass sheet according to the presentinvention is formed of a soda-lime-silica glass composition containing,expressed in wt. %, less than 0.020% of total iron oxide and 0.006 to2.0% of zinc oxide. In this specification, total iron oxide denotes anamount of iron oxide in terms of Fe₂O₃. All of the iron in thecomposition is counted as Fe₂O₃, even if it exists as FeO.

[0012] The soda-lime-silica glass composition constituting the hightransmittance glass sheet according to the present invention containsless than 0.020 wt. % (less than 200 ppm) of total iron oxide. Bymaintaining the content of total iron oxide at a low level as describedabove, it is made easier to obtain a high transmittance glass sheethaving, on a 4.0 mm thickness basis, a solar radiation transmittance of87.5% or higher. Preferably, total iron oxide is contained in an amountof not less than 0.005 wt. % as will be described later.

[0013] For the effective suppression of NiS formation in asoda-lime-silica glass composition containing less than 200 ppm of totaliron oxide, zinc oxide should be contained, in terms of ZnO, in anamount of not less than 0.006 wt. % (not less than 60 ppm). The additionof zinc oxide does not cause an increase in light absorption in thevisible light region. It has been found to be desirable for thesuppression of NiS formation that the content of zinc oxide be increasedin inverse proportion to the content of total iron oxide. When thecontent of total iron oxide has a value near 200 ppm, it is requiredthat ZnO be contained in an amount of not less than 60 ppm. When thecontent of total iron oxide is 50 ppm, preferably, ZnO is contained inan amount of not less than 180 ppm. More preferably, when the content oftotal iron oxide has a value near 200 ppm, ZnO is contained in an amountof not less than 100 ppm, and when the content of total iron oxide is 50ppm, ZnO is contained in an amount of not less than 300 ppm.

[0014] In manufacturing a high transmittance glass, in order to preventZnO from volatilizing during melting to damage a furnace, ZnO should becontained in an amount of not more than 2.0 wt. % (not more than 20,000ppm). In the case where a float bath is used for forming a glass sheet,in order to prevent ZnO that has volatilized and condensed in the floatbath from dropping onto a glass ribbon to form a defect, ZnO is useddesirably in an amount of not more than 5,000 ppm, and more desirably inan amount of not more than 1,000 ppm.

[0015] This problem, which is caused by dropping of a condensed materialthat has volatilized, does not occur in the case where a glass sheet ismanufactured, instead of using the float bath, for example, by a rollout process in which molten glass is rolled using a roller with anuneven (a predetermined pattern) or an even surface, and by a process inwhich molten glass that has been allowed to pass through a slit oroverflow from a melting tub is drawn.

[0016] Thus, as shown in FIG. 1, where an x-coordinate axis indicatesthe content of the total iron oxide expressed in ppm and a y-coordinateaxis indicates the content of the zinc oxide expressed in ppm, the glasscomposition has contents of the total iron oxide and the zinc oxidewhose values fall preferably within a range defined by a square ABCDformed by connecting Point A (200, 60), Point B (200, 20,000), Point C(50, 20,000), and Point D (50, 180) in this order, more preferablywithin a range defined by a square A′BCD′ formed by connecting Point A′(200, 100), Point B (200, 20,000), Point C (50, 20,000), and Point D′(50, 300) in this order, and most preferably within a range defined by asquare A′B′C′D′ formed by connecting Point A′ (200, 100), Point B′ (200,5,000), Point C′ (50, 5,000), and Point D′ (50, 300) in this order.

[0017] The present invention also provides a method of suppressingformation of nickel sulfide in a high transmittance glass sheet having asolar radiation transmittance of 87.5% or higher and/or a visible lighttransmittance of 90.0% or higher on a basis of a 4.0 mm thick glasssheet. In the method, a glass raw material is prepared so that a contentof total iron oxide in terms of Fe₂O₃ is less than 0.020 wt % and acontent of zinc oxide is 0.006 to 2.0 wt. %, and the glass raw materialis melted.

[0018] The content of zinc oxide required to suppress the formation ofnickel sulfide particles in a glass composition increases as the contentof total iron oxide is decreased when the content of the total ironoxide in the glass is in the range of 0.006 to 0.060 wt. %. Since zincoxide materials are costly compared with other raw materials, it wouldbe cost effective to use zinc oxide in the least possible amountrequired to suppress the formation of nickel sulfide particles.Therefore, in manufacturing soda-lime glasses successively, when thecontent of total iron oxide in a glass composition is decreased overtime, preferably, the content of zinc in the glass composition isincreased accordingly within the range of 0.006 to 0.50 wt. % (60 to5,000 ppm). Conversely, when the content of the total iron oxide in theglass composition is increased over time, preferably, the content ofzinc in the glass composition is decreased accordingly in the aboverange.

[0019] Examples of zinc compounds for zinc oxide (ZnO) that should beadded to a raw material include an inorganic zinc compound such as zincnitrate (Zn(NO₃)₂.6H₂O), zinc sulfate (ZnSO₄.7H₂O), a zinc halide (e.g.zinc fluoride (ZnF₂.4H₂O), zinc bromide (ZnBr₂), zinc chloride (ZnCl₂)and zinc iodide (ZnI₂)) and zinc phosphate (Zn₃(PO₄)₂.4H₂O); and anorganic zinc compound such as zinc benzoate (Zn(C₆H₅CO₂)₂) and zincacetate (Zn(CH₃CO₂)₂.2H₂O). Although these zinc compounds havesubstantially the same effects, it is most preferable to use at leastone selected from zinc nitrate and zinc sulfate from the viewpoint ofcost effectiveness or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a graph showing a preferred relationship between thecontent of total iron oxide and the content of zinc oxide in a glasscomposition according to the present invention.

[0021]FIG. 2 is a graph showing a relationship between the respectivecontents of T—Fe₂O₃ and CeO₂ and a fluorescence intensity ratio.

[0022]FIG. 3 is a graph showing a relationship between the content ofFe₂O₃ and a number of NiS particles formed in a soda-lime-silica glass.

[0023]FIG. 4 is a graph showing a relationship between an amount of Niadded and the content of ZnO required to reduce the number of NiSparticles to half in a soda-lime-silica glass.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The high transmittance glass sheet according to the presentinvention is formed of a glass composition containing total iron oxideand zinc oxide as described above. In the following description, theglass composition will be explained in greater detail.

[0025] Preferably, the high transmittance glass sheet according to thepresent invention has the following features. That is, the glass sheetis formed of a soda-lime-silica glass composition that contains inaddition to the zinc oxide, expressed in wt. %, 0.005 to less than0.020% of total iron oxide (hereinafter, referred to as T—Fe₂O₃) interms of Fe₂O₃, less than 0.008% of FeO, and 0 to 0.25% of cerium oxide,and has a ratio (hereinafter, referred to as a FeO ratio) of the contentof FeO in terms of Fe₂O₃ to the content of T—Fe₂O₃ of lower than 40%.When measurements are made on a 4.0 mm thickness basis, the hightransmittance glass sheet preferably has a solar radiation transmittanceof 87.5% or higher, a visible light transmittance as determined usingthe CIE standard illuminant C of 90.0% or higher, a dominant wavelengthas determined using the illuminant C of 540 to 580 nm, and an excitationpurity as determined using the illuminant C of 0.35% or lower. Here, thecontent (wt. %) of the zinc oxide is expressed by a value of an amountof the zinc oxide added with respect to a total amount of 100 wt. % ofthe other components.

[0026] More preferably, the high transmittance glass sheet has thefollowing features. That is, the glass sheet is formed of a compositionthat is substantially free from cerium oxide (the content of CeO₂ isless than 0.005 wt. %) and has a FeO ratio of equal to or higher than22% to lower than 40%. In this case, when a measurement is made on a 4.0mm thickness basis, the high transmittance glass sheet has an excitationpurity as determined using the illuminant C of 0.25% or lower. Thiscomposition allows a high transmittance and extremely light coloredglass sheet.

[0027] Furthermore, a high transmittance glass sheet that is formed of aglass composition containing 0 to 0.005 wt. % of cerium oxide, not morethan 0.03 wt. % of manganese oxide, and not more than 0.01 wt. % ofvanadium oxide can achieve the following. That is, when the hightransmittance glass sheet is exposed to ultraviolet radiation at awavelength of not more than 400 nm, for example, ultraviolet irradiationaccording to the light stability test specified in the JapaneseIndustrial Standards, R3212, on a 4.0 mm thickness basis, thetransmittance (in the near-infrared region) at a wavelength of 1,000 nmcan be improved by not less than 0.1%, and in some cases, by not lessthan 0.3% with respect to that of the glass sheet before being exposedto the ultraviolet radiation. Furthermore, after the ultravioletirradiation, the solar radiation transmittance and the visible lighttransmittance of the high transmittance glass sheet also can beincreased to 90.0% or higher and 90.5% or higher, respectively. Althoughnot entirely clarified, conceivably, lowering of the FeO ratiocontributes to these improvements in the transmittance in thenear-infrared region. For example, even when a glass sheet has a FeOratio of 22% or higher, ultraviolet irradiation allows the FeO ratio tobe lowered by 3 to 5%, so that the content of FeO can be reduced tolower than 22%.

[0028] Furthermore, it also is more preferable that the hightransmittance glass sheet has the following features. That is, the hightransmittance glass sheet is formed of a composition that contains,expressed in wt. %, 0.02 to 0.25% of cerium oxide, and has a FeO ratioof lower than 22%. When measurements are made on a 4.0 mm thicknessbasis, the high transmittance glass sheet has a solar radiationtransmittance of 90.0% or higher and a visible light transmittance asdetermined using the illuminant C of 90.5% or higher. This allows a hightransmittance glass sheet to be obtained that exhibits a hightransmittance particularly in a region ranging from the visible regionto the near-infrared region.

[0029] Furthermore, particularly for the efficient conversion ofultraviolet light into visible light, a high transmittance glass sheetis preferred that contains, expressed in wt. %, 0.025 to 0.20% of ceriumoxide, and has a ratio of a fluorescence intensity at a wavelength of395 nm to a fluorescence intensity at a wavelength of 600 nm (f(395nm)/f(600 nm), hereinafter, referred to also as a fluorescence intensityratio) of 10 or higher when subjected to ultraviolet irradiation at awavelength of 335 nm. Furthermore, a high transmittance glass sheet isdesired that contains 0.03 to 0.15 wt. % of cerium oxide and has afluorescence intensity ratio of 15 or higher. Moreover, a hightransmittance glass sheet is desired most that contains 0.05 to 0.12 wt.% of cerium oxide and has a fluorescence intensity ratio of 25 orhigher, since the glass sheet allows most efficient conversion ofultraviolet light into visible light.

[0030] Preferably, the above-mentioned soda-lime-silica glasscomposition according to the present invention contains, in addition tothe iron oxide, the zinc oxide and the cerium oxide that are describedabove, as components constituting a base glass composition, expressed inwt. %, 65 to 80% of SiO₂, 0 to 5% of Al₂O₃, 0 to 7% of MgO, 5 to 15% ofCaO, where a total content of MgO and CaO is more than 7% and not morethan 17%, 10 to 18% of Na₂O, 0 to 5% of K₂O, where a total content ofNa₂O and K₂O is 10 to 20%, and 0.05 to 0.3% of SO₃. The content of theabove-mentioned zinc oxide is expressed by an amount of the zinc oxideadded with respect to a total amount of 100% of the above componentsconstituting the base glass composition.

[0031] Furthermore, more preferably, the total content of MgO and CaO(MgO+CaO) is 10 to 17 wt. %, and the content of S03 is 0.08 to 0.15 wt.%. Moreover, it is desirable that the content of MgO be 0.5 to 7 wt. %since this allows meltability and formability to be improved.Furthermore, it is desirable that the content of Al₂O₃ be 0.5 to 5 wt. %since this allows water resistance to be improved.

[0032] Hereinafter, the composition of the high transmittance glasssheet according to the present invention will be described in terms ofthe components other than the zinc oxide described earlier. In thefollowing description, the respective contents of the components areexpressed in wt. %.

[0033] In glass, iron oxide is present in forms of Fe₂O₃ and FeO. Fe₂O₃serves to enhance an ultraviolet-absorbing ability, and FeO serves toenhance a heat-absorbing ability. In order to attain a hightransmittance as desired, preferably, the content of T—Fe₂O₃ (a totalcontent of Fe₂O₃ and FeO in terms of Fe₂O₃) is less than 0.020%, andpreferably, the content of FeO is less than 0.008%, and the FeO ratio islower than 40%. When the contents of T—Fe₂O₃ and FeO and the FeO ratioreach and become greater than their respective upper limits, the visiblelight transmittance becomes too low, and the glass takes more on a bluetone of FeO.

[0034] When the content of T—Fe₂O₃ is less than 0.005%, it is necessaryto use high purity materials having low iron contents. This leads to asubstantial cost increase. Thus, preferably, T—Fe₂O₃ is contained in anamount of not less than 0.005%.

[0035] When used for a glass substrate and a cover glass plate for asolar cell panel using amorphous silicon, a glass sheet preferably has ahigh transmittance with respect to light having a wavelength in thevicinity of 500 to 600 nm and exhibits moderate solar radiationabsorption. In this case, preferably, when the content of T—Fe₂O₃ is inthe above-mentioned range, the content of FeO is more than 0.003% andless than 0.008%, and the FeO ratio is equal to or higher than 22% andlower than 40%.

[0036] When used for a glass substrate and a cover glass plate for asolar cell panel using crystalline silicon, preferably, a glass sheethas a high transmittance with respect to light having a wavelength inthe vicinity of 1,000 nm. In this case, preferably, when the content ofT—Fe₂O₃ is in the above-mentioned range, the content of FeO is less than0.004%, and the FeO ratio is lower than 22%.

[0037] Cerium oxide (CeO₂) is effective in regulating the content of FeOand the FeO ratio. Particularly, in order to attain a low FeO contentand a low FeO ratio required when a high transmittance at a wavelengthin the vicinity of 1,000 nm is desired, preferably, CeO₂ is added in anamount of 0.02 to 0.25%.

[0038] Furthermore, with respect to glasses containing 0.005 to 0.08 wt.% of T—Fe₂O₃ and 0 to 0.20 wt. % of CeO₂, a relationship between thecontent of CeO₂ and a fluorescence property is shown in FIG. 2. As shownin FIG. 2, it was found that ultraviolet light was absorbed andconverted to visible light most effectively when the content of CeO₂ wasin a given range. That is, it was found that a high transmittance glasssheet could be obtained that contained less than 0.06% of T—Fe₂O₃ and0.025 to 0.20% of CeO₂, thereby achieving a fluorescence intensity ratioof 10 or higher, a fluorescence intensity ratio of 15 or higher when thecontent of CeO₂ was 0.03 to 0.15%, and a fluorescence intensity ratio of25 or higher when the content of CeO₂ was 0.05 to 0.12%.

[0039] The high transmittance glass sheet described above is suitablefor use for an interior material, a glass for a showcase or the likeparticularly because the glass sheet takes on a fluorescent color withgradations when ultraviolet light is incident on an edge surface of theglass sheet from a cross sectional direction.

[0040] Furthermore, when used for a substrate and a cover glass platefor a solar cell panel or the like, the above-mentioned hightransmittance glass sheet is used most suitably since the glass sheetallows energy in the ultraviolet region that hardly contributes to powergeneration to be converted into light in the visible region, therebyallowing the power generation efficiency to be enhanced.

[0041] SiO₂ is a main component forming a skeleton of the glass. Whenthe content of SiO₂ is less than 65%, the durability of the glass isdecreased, and when the content of SiO₂ is more than 80%, melting of theglass is hindered.

[0042] Although not an indispensable component, Al₂O₃ serves to improvethe durability and the water resistance of the glass. When the contentof Al₂O₃ is increased, melting of the glass is hindered. Thus, thecontent of Al₂O₃ should be 0 to 5%. In order to improve the durabilityand the water resistance, preferably, the content of Al₂O₃ is not lessthan 0.5%. In order not to impair the meltability of the glass,preferably, the content of Al₂O₃ is not more than 2.5%. More preferably,the content of Al₂O₃ is in the range of 1.0 to 2.5%.

[0043] Both MgO and CaO serve to improve the durability of the glass andregulate the liquidus temperature and the viscosity of the glass in aforming process. Although not an indispensable component, MgO allows alow liquidus temperature to be maintained when contained in a moderateamount. Thus, the content of MgO is preferably more than 0.5%, and morepreferably not less than 2%. When the content of MgO exceeds 7%, theliquidus temperature is increased excessively. On the other hand, whenthe content of CaO is less than 5%, the meltability is degraded.Further, when the content of CaO exceeds 15%, the liquidus temperatureis increased. Thus, more preferably, the content of CaO is not more than13%. When a total content of MgO and CaO is not more than 7%, thedurability of the glass is decreased. Conversely, when the total contentexceeds 17%, the liquidus temperature is increased. Thus, morepreferably, the total content is not more than 15%. In the case wherethe total content of MgO and CaO is as small as, for example, less than10%, it is required that the content of Na₂O be increased so that thedegradation of the meltability and an increase in viscosity of a meltare compensated. This leads to a cost increase and a decrease inchemical durability of the glass. Thus, more desirably, the totalcontent of MgO and CaO is not less than 10%.

[0044] Both Na₂O and K₂O serve to accelerate melting of the glass. Whenthe content of Na₂O is less than 10% or when a total content of Na₂O andK₂O is less than 10%, only a poor effect of accelerating glass meltingcan be obtained. It is not preferable that the content of Na₂O exceeds18% or the total content of Na₂O and K₂O exceeds 20% since this resultsin a decrease in the durability of the glass. In applications wherewater resistance is required particularly, the content of Na₂O ispreferably not more than 15%, and more desirably not more than 14.5%.Since a material cost of K₂O is high compared with Na₂O, K₂O is not anindispensable component. Even when K₂O is used, it is not preferablethat the content of K₂O exceeds 5%.

[0045] SO₃ serves to accelerate clarification of the glass. When thecontent of SO₃ is less than 0.05%, a sufficient clarifying effect cannotbe attained by a regular melting method. Thus, desirably, the content ofSO₃ is more than 0.1%. Conversely, when the content of SO₃ exceeds 0.3%,SO₂ produced as a result of decomposition of SO₃ remains in the glass inthe form of a bubble, and SO₃ dissolved in the glass becomes more likelyto produce bubbles by reboiling.

[0046] Although not an indispensable component, TiO₂ can be added in aproper amount for the purposes of enhancing an ultraviolet-absorbingability or the like as long as the amount is in the range that allowsthe optical properties that are the intended properties of the presentinvention not to be impaired. When an excessive amount of TiO₂ iscontained, the glass becomes more likely to be yellowish, and thetransmittance at a wavelength in the vicinity of 500 to 600 nm islowered. Thus, desirably, the content of TiO₂ is limited to a low levelin the range of less than 0.2%.

[0047] Furthermore, even when fluorine, boron oxide, barium oxide, andstrontium oxide are contained, the effect of the present invention isnot impaired. However, these components create adverse impacts such as acost increase, shortening a furnace life, release of harmful substancesinto the air or the like. Thus, the glass composition should besubstantially free from these components.

[0048] As a component to be added as an oxidizing agent to a glasshaving the above-mentioned composition, preferably, cerium oxide in anamount in the range defined in the above description is used, in view ofthe effect of cerium oxide and an ultraviolet-absorbing effect asanother particular effect of cerium oxide. However, an oxidizing agentother than cerium oxide, for example, manganese oxide may be added in anamount in the range of not more than 1% in combination with cerium oxideor as an only oxidizing agent.

[0049] Furthermore, SnO₂ may be added as a reducing agent in an amountin the range of not more than 1%. Moreover, as in a general case, inaddition to the iron oxide, the cerium oxide and the manganese oxidethat are described above, concurrently with the addition of thesecomponents, at least one selected from the group consisting of Se, CoO,Cr₂O₃, NiO, V₂O₅, MoO₃ or the like may be added as a coloring agent, inan amount in the range that allows the high transmittance that is theintended property of the present invention not to be impaired. However,when the coloring agent is added in an excessive amount, a color tone isintensified and the visible light transmittance is lowered. Thus,desirably, these compounds are not added practically. For example,desirably, the content of V₂O₅ is not more than 0.01 wt. %.

[0050] The effect of the high transmittance glass according to thepresent invention can be attained effectively when the hightransmittance glass is subjected to quenching (tempering).

[0051] The high transmittance glass sheet according to the presentinvention is highly demanded particularly in use for a solar cell panel.When the glass sheet is used for a solar cell panel, an anti-reflectingfilm and a conductive film can be formed on the glass sheet. Even whenthese films are formed on the glass sheet, the glass properties are notaffected. Further, regardless of whether these films are formed, theglass sheet can be subjected to processing involving heating such astempering and bending. Generally, in rapidly cooling for tempering, thehigh transmittance glass sheet is heated to a temperature near thesoftening point of the glass sheet, and then cooled rapidly by beingbrought into contact with cold air or other fluids.

[0052] The high transmittance glass sheet according to the presentinvention generally has a thickness of 0.3 mm to 30 mm and is suitablefor use for an interior glass, a showcase, a display case, a hightransmittance non-colored window glass, a high transmittance non-coloredmirror, a glass substrate for a solar cell panel, a cover glass platefor a solar cell panel, a solar water heater, a high solar-heattransmittance window glass, a window glass for a microwave oven, or aflat display substrate glass such as a front panel or the like.

EXAMPLE

[0053] With respect to soda-lime-silica glass in which Ni metal iscontained intentionally, a relationship between the content of totaliron oxide (in terms of Fe₂O₃) and the degree to which NiS is likely tobe formed was examined. As can be seen from the results of theexamination shown in FIG. 3, it was observed that NiS became more likelyto be formed as the content of the total iron oxide was decreased from0.20 wt. %, and in particular, the number of NiS particles formedincreased steeply when the content of the total iron oxide was not morethan 0.060 wt. %. Table 1 shows the respective values of the content ofthe total iron oxide, an amount of the Ni metal added, the number of theNiS particles formed, and the maximum diameter of the NiS particles thatwere used to obtain the results shown in FIG. 3. These results wereobtained by using a crucible having a capacity of 250 cm³. Also inactual melting and forming operations of soda-lime-silica glass using atank-type melting furnace, it was confirmed that a ratio of defects in atempered glass sheet caused by soaking increased as the content of ironoxide in the glass was decreased from 0.20 wt. %. TABLE 1 Sample 1Sample 2 Sample 3 Content of total 0.018 0.050 0.200 iron oxide (wt. %)Content of Ni* 700 700 700 (ppm) NiS (no. of particles/ 323 113 50 100 gof glass) Maximum diameter 150 120 120 of NiS particle (μm)

[0054] Two types of raw materials varying in Fe₂O₃ content were preparedby mixing reagent chemicals or equivalents of SiO₂, Al₂O₃, MgO, CaCO₃,Na₂CO₃, K₂CO₃, TiO₂, Na₂SO₄, Fe, and carbon (C). In Table 2, CompositionNo. 1 represents soda-lime-silica glass having a Fe₂O₃ content of lessthan 0.02 wt. %, and Composition No. 2 represents soda-lime-silica glasshaving a Fe₂O₃ content of 0.05 wt. %. A relationship between therespective amounts of Na₂SO₄ and carbon that were used and an amount ofresultant SO₃ was determined beforehand. In each composition, based onthe relationship, an amount of Na₂SO₄ in terms of Na₂O to be added wasset to 0.74 wt. % so that the amount of resultant SO₃ shown in Table 2was attained. The amount of Na₂O was regulated so that the amount shownin Table 2 was attained by using Na₂CO₃. In Table 2, the respectivecontents are expressed in wt. %. TABLE 2 Composition No. 1 CompositionNo. 2 SiO₂ 71.9 71.8 Al₂O₃ 1.9 1.9 MgO 3.9 3.9 CaO 8.2 8.2 Na₂O 13.613.6 K₂O 0.3 0.3 TiO₂ 0.030 0.030 CeO₂ 0 0 MnO₂ 0 0 SO₃ 0.200 0.200T—Fe₂O₃ 0.018 0.050 FeO in T—Fe₂O₃ 0.005 0.015 Total 100.0 100.0

[0055] Glass batch materials of Samples 4 to 47 were prepared in thefollowing manner. That is, with respect to each of these two types ofraw materials, a powder of Ni metal having a particle diameter of 149 μmand a powder of zinc nitrate (Zn(NO₃)₂.6H₂O) or zinc sulfate(ZnSO₄.7H₂O) are added in the respective amounts shown in Tables 3 and4. In the tables, A and B in the column titled “additive” represent zincsulfate (ZnSO₄ .7H₂O) and zinc nitrate (Zn(NO₃)₂.6H₂O), respectively.

[0056] With respect to each sample, a batch of these materials was putin an alumina crucible having a capacity of 250 cc and preheated at atemperature of 600° C. for 30 minutes. Then, the batch was inserted inan electric furnace maintained at a temperature of 1,370° C., and thetemperature of the electric furnace was raised to 1,400° C. in 10minutes. After being maintained at this temperature for 2.2 hours, thebatch was taken out of the electric furnace and cast out to be annealedto room temperature, so that a glass sheet was obtained.

[0057] With respect to each glass obtained, the number of NiS particleswas measured using a stereomicroscope. The results of the measurementsare shown in Tables 3 and 4. TABLE 3 NiS Amount of (no. of Compo- Amountof additive in particles/ sition Ni added terms of ZnO 100 g of No.(ppm) Additive* (ppm) glass) Sample 4 1 350 A 0 300 Sample 5 1 350 A 100270 Sample 6 1 350 A 200 290 Sample 7 1 350 A 300 220 Sample 8 1 350 A400 230 Sample 9 1 350 A 1,000 110 Sample 10 1 140 A 0 52 Sample 11 1140 A 200 24 Sample 12 1 35 A 0 4 Sample 13 1 35 A 200 3 Sample 14 1 35A 300 2 Sample 15 1 35 A 400 0 Sample 16 1 140 B 0 52 Sample 17 1 140 B300 9 Sample 18 2 350 B 0 57 Sample 19 2 350 B 27 52 Sample 20 2 350 B68 46 Sample 21 2 350 B 103 40 Sample 22 2 350 B 205 33 Sample 23 2 350B 410 4 Sample 24 2 175 B 0 27 Sample 25 2 175 B 27 19

[0058] TABLE 4 NiS Amount of (no. of Compo- Amount of additive inparticles/ sition Ni added terms of ZnO 100 g of No. (ppm) Additive*(ppm) glass) Sample 26 2 175 B 68 17 Sample 27 2 175 B 103 14 Sample 282 175 B 205 10 Sample 29 2 87.5 B 0 13 Sample 30 2 87.5 B 27 8 Sample 312 87.5 B 68 2 Sample 32 2 87.5 B 103 1 Sample 33 2 87.5 B 205 0 Sample34 2 350 A 0 57 Sample 35 2 350 A 50 54 Sample 36 2 350 A 126 50 Sample37 2 350 A 189 46 Sample 38 2 350 A 378 35 Sample 39 2 350 A 756 14Sample 40 2 175 A 0 27 Sample 41 2 175 A 50 20 Sample 42 2 175 A 126 10Sample 43 2 175 A 189 2 Sample 44 2 175 A 378 2 Sample 45 2 87.5 A 0 15Sample 46 2 87.5 A 50 9 Sample 47 2 87.5 A 126 0

[0059] It can be seen from Tables 3 and 4 that in each of the glassescontaining 0.050 wt. % of T—Fe₂O₃ (Composition No. 2), by adding a traceamount of zinc nitrate (Zn(NO₃)₂.6H₂O) or zinc sulfate (ZnSO₄.7H₂O) tothe glass materials, a considerable effect of suppressing the formationof NiS in a glass product can be obtained. On the other hand, it can beseen from the tables that, in each of the glasses containing less than0.02 wt. % of T—Fe₂O₃ (Composition No. 1), an effect of preventing theformation of NiS cannot be obtained when the amount of zinc nitrate orzinc sulfate added is small, and can be obtained by increasing theamount of the zinc nitrate or the zinc sulfate to be added.

[0060] Based on the results shown in Tables 3 and 4, with respect toeach of glasses having the respective Fe₂O₃ contents, the rate of anamount of an additive in terms of ZnO at which the amount of NiS formedwas reduced to half was determined. The results are plotted in FIG. 4.As is apparent from FIG. 4, compared with the glass containing 0.050 wt.% of T—Fe₂O₃, in the glass containing 0.018 wt. % of T—Fe₂O₃ in order toreduce the amount of NiS formed to half, it is required that zincnitrate or zinc sulfate in terms of ZnO be used in a two- to four-foldamount, namely, of about not less than 100 ppm.

EXAMPLES 1 TO 18

[0061] Glass batch materials having compositions shown in Tables 5 to 7,in which the respective contents are expressed in terms of oxide and inwt. %, were prepared using low-iron silica sand, alumina, limestone,dolomite, soda ash, salt cake, magnesium oxide, cerium oxide, manganesedioxide, zinc sulfate (ZnSO₄.7H₂O), and a carbon-based reducing agent.Each batch of these materials was heated in an electric furnace to atemperature of 1,450° C. to be melted. After four hours of melting, thebatch was poured onto a stainless steel plate and annealed to roomtemperature, so that a glass sheet having a thickness of about 10 mm wasobtained. In the tables, the values of concentration are expressed inwt. %.

[0062] After that, the surface of the glass sheet was ground so that aglass sheet sample having a thickness of 4.0 mm was obtained. Withrespect to each sample thus obtained, measurements were performed usingthe illuminant C for optical properties that are a visible lighttransmittance, a dominant wavelength, an excitation purity, a solarradiation transmittance, and a fluorescence intensity ratio. Thefluorescence intensity ratio was determined in the following manner.That is, each of the samples described above was subjected toultraviolet irradiation at a wavelength of 335 nm, and a fluorescenceintensity was determined at the respective wavelengths. Then, acalculation was performed using the formula, fluorescence intensityratio =(fluorescence intensity at 395 nm/fluorescence intensity at 600nm) as an index of the fluorescence intensity. Further, water resistancewas evaluated by determining an elution amount of Na₂O (mg) according toJIS 3502. The respective values of the optical properties and the waterresistance of each sample as the results of the measurements are shownin Tables 5 to 7.

[0063] In the same manner as in the above cases of Examples 1 to 18, 18types of glass sheets (ZnO-added and Ni-added samples) that were about10 mm in thickness were obtained. However, for each of these glasssheets, in preparation of a raw material, a powder of Ni metal having aparticle diameter of 149 μm further was added in an amount of 150 ppmwith respect to a total amount of the raw material (in terms of oxide).Further, in the same manner as in the above cases of the Examples 1 to18, 18 types of glass sheets (ZnO-free and Ni-added samples) that wereabout 10 mm in thickness were obtained. However, for each of these glasssheets, in preparation of a raw material, zinc sulfate was not added,and a powder of Ni metal having a particle diameter of 149 μm was addedin an amount of 150 ppm with respect to a total amount of the rawmaterial (in terms of oxide).

[0064] With respect to these two sets of samples, measurements wereperformed for the number of NiS particles using a stereomicroscope. Asthe results of the measurements, in the samples to which ZnO was notadded and Ni was added, 30 to 50 NiS particles per 100 g of glass wereobserved, while in the samples to which ZnO was added and Ni was added,0 to 10 NiS particles per 100 g of glass were observed. TABLE 5 1 2 3 45 6 SiO₂ 71.1 704 698 697 68.0 715 Al₂O₃ 1.8 2.0 2.9 4.8 2.5 0.2 MgO 4.42.1 3.9 2.1 5.9 4.8 CaO 9.0 11.2 7.8 8.9 8.1 7.2 Na₂O 12.6 12.9 14.613.2 14.1 15.1 K₂O 0.8 1.1 0.7 0.9 0.9 0.9 SO₃ 0.23 0.22 0.28 0.09 0.120.14 T—Fe₂O₃ 0.019 0.019 0.018 0.018 0.016 0.016 TiO₂ 0.04 0.03 0.030.04 0.03 0.03 CeO₂ 0 0 0 0 0 0 MnO₂ 0 0 0 0 0 0 Total 100.0 100.0 100.0100.0 100.0 100.0 ZnO 0.010 0.010 0.015 0.015 0.020 0.020 FeO 0.0050.007 0.006 0.005 0.004 0.006 FeO ratio (%) 26 37 33 28 25 38 Visiblelight 914 90.8 91.1 91.4 91.5 90.9 transmittance (%) Solar radiation90.3 89.1 89.8 90.3 90.7 89.5 transmittance (%) Dominant 558 552 553 557562 552 wavelength (nm) Excitation 0.19 048 0.18 0.19 0.19 0.17 purity(%) Fluorescence intensity ratio 0 1 2 0 0 1 Water resist- 0.59 0.800.50 0.15 0.76 1.69 ance (mg)

[0065] TABLE 6 Example 7 8 9 10 11 12 SiO₂ 71.7 71.7 71.6 71.6 71.5 71.5Al₂O₃ 1.7 1.7 1.7 1.7 1.7 1.7 MgO 4.2 4.2 4.2 4.2 4.2 4.2 CaO 8.5 8.58.5 8.5 8.5 8.5 Na₂O 13.0 13.0 13.0 13.0 13.0 13.0 K₂O 0.7 0.7 0.7 0.70.7 0.7 SO₃ 0.12 0.12 0.12 0.12 0.12 0.12 T—Fe₂O₃ 0.015 0.015 0.0150.015 0.015 0.015 TiO₂ 0.02 0.02 0.02 0.02 0.02 0.02 CeO₂ 0 0.04 0.060.10 0.14 0.20 MnO₂ 0 0 0 0 0 0 Total 100.0 100.0 100.0 100.0 100.0100.0 ZnO 0.040 0.040 0.040 0.040 0.040 0.040 FeO 0.004 0.003 0.0030.002 0.002 0.001 FeO ratio (%) 27 20 20 13 13 7 Visible light 91.2 91.691.6 91.7 91.6 91.6 transmittance (%) Solar radiation 90.0 90.7 90.691.0 91.0 91.3 transmittance (%) Dominant 554 565 565 570 571 573wavelength (nm) Excitation 0.19 0.20 0.20 0.20 0.24 0.30 purity (%)Fluorescence intensity ratio 2 21 31 28 16 11 Water resist- 0.58 0.580.58 0.58 0.59 0.59 ance (mg)

[0066] TABLE 7 13 14 15 16 17 18 SiO₂ 71.0 71.7 71.6 72.0 71.1 71.1Al₂O₃ 1.4 1.7 1.7 1.7 1.8 1.5 MgO 4.3 4.0 4.2 4.2 4.4 6.2 CaO 8.6 8.58.5 8.5 9.0 8.7 Na₂O 13.5 13.0 13.0 12.5 12.6 11.1 K₂ 0.7 0.7 0.7 0.70.7 1.0 SO₃ 0.22 0.23 0.20 0.21 0.23 0.23 T—Fe₂O₃ 0.019 0.019 0.0110.011 0.013 0.013 TiO₂ 0.03 0.03 0.04 0.04 0.04 0.04 CeO₂ 0.22 0.10 0.050.06 0.10 0.10 MnO₂ 0 0.06 0 0.08 0 0 Total 100.0 100.0 100.0 100.0100.0 100.0 ZnO 0.020 0.020 0.050 0.050 0.040 0.40 FeO 0.001 0.002 0.0020.001 0.002 0.002 FeO ratio (%) 5 11 18 9 15 15 Visible light 91.6 91.691.7 91.8 91.7 91.7 transmittance (%) Solar radiation 91.2 91.0 91.091.3 90.9 90.9 transmittance (%) Dominant 573 570 567 570 568 568wavelength (nm) Excitation 0.31 0.23 0.20 0.21 0.20 0.20 purity (%)Fluorescence intensity ratio 9 26 27 27 28 28 Water resist- 0.79 0.570.52 0.44 0.53 0.44 ance (mg)

[0067] As discussed in the foregoing description, according to thepresent invention, 0.006 to 0.20 wt. % of zinc oxide is contained in asoda-lime-silica glass containing total iron oxide in terms of Fe₂O₃ inan amount of less than 0.02 wt. %, and thus a sufficient effect ofreducing or eliminating the formation of NiS particles can be attained,thereby allowing an improved quality glass product to be obtained.

[0068] Furthermore, the addition of the zinc oxide has almost noinfluence on visible light transmittance and ultraviolet transmittance,and also has no influence on physical property values of the glass interms of a coloring property, viscosity, expansion or the like. Thus,particularly, the general glass quality can be maintained, whilesecuring high transmittance, thereby achieving a substantial advantagefrom a practical viewpoint. Furthermore, the present invention allowsmanufacturing of glass products containing almost no NiS. Thus, also ina process of manufacturing a tempered glass, a heating (soaking) processfor removing glasses containing NiS can be omitted after a quenchtempering process, thereby allowing a manufacturing cost to be reduced.Further, a rate of glass breakage caused in soaking can be lowered,thereby allowing product yield to be improved.

[0069] The invention may be embodied in other forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A high transmittance glass sheet formed of asoda-lime-silica glass composition comprising, expressed in wt. %, lessthan 0.020% of total iron oxide in terms of Fe₂O₃ and 0.006 to 2.0% ofzinc oxide.
 2. The high transmittance glass sheet according to claim 1,wherein where an x-coordinate axis indicates a content of the total ironoxide expressed in ppm and a y-coordinate axis indicates a content ofthe zinc oxide expressed in ppm, the glass composition has contents ofthe total iron oxide and the zinc oxide whose values fall within a rangedefined by a square ABCD formed by connecting Point A (200, 60), Point B(200, 20,000), Point C (50, 20,000), and Point D (50, 180) in thisorder.
 3. The high transmittance glass sheet according to claim 1,wherein the glass composition comprises, expressed in wt. %, not lessthan 0.005% to less than 0.020% of the total iron oxide, less than0.008% of FeO, and 0 to 0.25% of cerium oxide; the glass composition hasan FeO ratio of lower than 40%, where the FeO ratio is a ratio of acontent of FeO in terms of Fe₂O₃ to a content of the total iron oxide;and the glass sheet has, on a 4.0 mm thickness basis, a solar radiationtransmittance of 87.5% or higher, a visible light transmittance of 90.0%or higher, a dominant wavelength of 540 to 580 nm, and an excitationpurity of 0.35% or lower, where the visible light transmittance, thedominant wavelength and the excitation purity are measured withilluminant C.
 4. The high transmittance glass sheet according to claim3, wherein the glass composition comprises, expressed in wt. %, 0 to0.005% of cerium oxide and has a FeO ratio of equal to or higher than22% to lower than 40%; and the glass sheet has, on the 4.0 mm thicknessbasis, an excitation purity of 0.25% or lower, where the excitationpurity is measured with the illuminant C.
 5. The high transmittanceglass sheet according to claim 3, wherein the glass compositioncontains, expressed in wt. %, 0.02 to 0.25% of cerium oxide and has aFeO ratio of lower than 22%; and the glass sheet has, on the 4.0 mmthickness basis, a solar radiation transmittance of 90.0% or higher anda visible light transmittance of 90.5% or higher, where the visiblelight transmittance is measured with the illuminant C.
 6. The hightransmittance glass sheet according to claim 3, wherein the glasscomposition contains, expressed in wt. %, 0 to 0.005% of cerium oxide,not more than 0.03% of manganese oxide, and not more than 0.01% ofvanadium oxide.
 7. The high transmittance glass sheet according to claim6, wherein by ultraviolet radiation at a wavelength of not more than 400nm, a transmittance at a wavelength of 1,000 nm of the glass sheet isincreased, on the 4.0 mm thickness basis, by not less than 0.1% withrespect to the transmittance of the glass sheet before being exposed tothe ultraviolet radiation.
 8. The high transmittance glass sheetaccording to claim 6, wherein after being exposed to ultravioletradiation at a wavelength of not more than 400 nm, the glass sheet has aFeO ratio of lower than 22%.
 9. The high transmittance glass sheetaccording to claim 6, wherein when subjected to ultraviolet irradiationaccording to a light stability test specified in Japanese IndustrialStandards, R3212, the glass sheet has, on the 4.0 mm thickness basis, atransmittance at a wavelength of 1,000 nm that is increased by not lessthan 0.3% compared with the transmittance of the glass sheet beforebeing subjected to the ultraviolet irradiation; and after theultraviolet irradiation, the glass sheet has a solar radiationtransmittance of 90.0% or higher and a visible light transmittance of90.5% or higher.
 10. The high transmittance glass sheet according toclaim 3, wherein the glass composition comprises 0.025 to 0.20 wt. % ofcerium oxide; and the glass sheet has a fluorescence intensity ratio of10 or higher when subjected to ultraviolet irradiation at a wavelengthof 335 nm, where the fluorescence intensity ratio is a ratio of afluorescence intensity at a wavelength of 395 nm to a fluorescenceintensity at a wavelength of 600 nm.
 11. The high transmittance glasssheet according to claim 1, wherein the glass composition furthercomprises, expressed in wt. %: 65 to 80% of SiO₂, 0 to 5% of Al₂O₃, 0 to7% of MgO, 5 to 15% of CaO, where a total content of MgO and CaO is morethan 7% and not more than 17%, 10 to 18% of Na₂O, 0 to 5% of K₂O, wherea total content of Na₂O and K₂O is 10 to 20%, and 0.05 to 0.3% of SO₃.12. The high transmittance glass sheet according to claim 11, whereinthe glass composition is substantially free from fluorine, boron oxide,barium oxide, and strontium oxide.
 13. The high transmittance glasssheet according to claim 11, wherein the glass composition issubstantially free from Se, CoO, Cr₂O₃, NiO, V₂O₅ and MoO₃.
 14. The hightransmittance glass sheet according to claim 1, wherein the glass sheetis a tempered glass.
 15. A method of manufacturing a high transmittanceglass sheet as claimed in claim 1, comprising: adding a zinc compound toa glass raw material so that a content of zinc oxide in the hightransmittance glass sheet is 0.006 to 2.0 wt. %; melting the glass rawmaterial; and forming the high transmittance glass sheet, wherein thezinc compound is at least one selected from zinc nitrate and zincsulfate.
 16. The method according to claim 15, further comprisingtempering the high transmittance glass sheet.
 17. A method ofsuppressing formation of nickel sulfide in a high transmittance glasssheet, the high transmittance glass sheet having a solar radiationtransmittance of 87.5% or higher and/or a visible light transmittance of90.0% or higher on a basis of a 4.0 mm thick glass sheet, the methodcomprising: preparing a glass raw material so that a content of totaliron oxide in terms of Fe₂O₃ is less than 0.020 wt % and a content ofzinc oxide is 0.006 to 2.0 wt. %; and melting the glass raw material.18. A method of manufacturing a soda-lime glass that allows formation ofnickel sulfide particles in a glass formed by melting to be suppressedby addition of a zinc compound to a glass raw material, wherein when acontent of total iron oxide (in terms of Fe₂O₃) in a glass is increasedor decreased from a predetermined value, within a range of 0.005 wt. %to 0.06 wt. %, a content of the zinc oxide is decreased or increasedfrom a predetermined value according to an increase or a decrease in thecontent of the total iron oxide, within a range of 0.006 to 2.0 wt. %,whereby the glass exhibits high transmittance with a visible lighttransmittance of 90.0% or higher on a basis of a 4.0 mm thick glasssheet, while suppressing formation of nickel sulfide particles in theglass.